Imaging devices, including charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) imagers, are commonly used in photo-imaging applications.
A CMOS imager circuit includes a focal plane array of pixel cells. Each one of the cells includes a photoconversion device or photosensor such as, for example, a photogate, photoconductor, or photodiode, for generating and accumulating photo-generated charge in a portion of the substrate of the array. A readout circuit is connected to each pixel cell and includes at least an output transistor, which receives photo-generated charges from a doped diffusion region and produces an output signal that is read-out through a pixel access transistor.
One typical CMOS imager pixel circuit, the three-transistor (3T) pixel, contains a photosensor for supplying photo-generated charge to a diffusion region; a reset transistor for resetting the diffusion region; a source follower transistor having a gate connected to the diffusion region, for producing an output signal; and a row select transistor for selectively connecting the source follower transistor to a column line of a pixel array. Another typical CMOS imager pixel employs a four-transistor (4T) configuration, which is similar to the 3T configuration, but utilizes a transfer transistor to gate charges from the photosensor to the diffusion region and the source follower transistor for output.
Exemplary CMOS imaging circuits, processing steps thereof, and detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. No. 6,140,630 to Rhodes, U.S. Pat. No. 6,376,868 to Rhodes, U.S. Pat. No. 6,310,366 to Rhodes et al., U.S. Pat. No. 6,326,652 to Rhodes, U.S. Pat. No. 6,204,524 to Rhodes, and U.S. Pat. No. 6,333,205 to Rhodes. The disclosures of each of the forgoing are hereby incorporated by reference herein in their entirety.
There are two general types of imagers. The first type combines a low capacitance photosensor and a low capacitance floating diffusion region. This combination results in an imager with high conversion gain and excellent low light sensitivity, but poor dynamic range and low fill well capacity. A second type of imager combines a high charge capacity photosensor with a high charge capacity floating diffusion region to achieve high full well capacity and dynamic range. This imager, however, has a low conversion gain and poor low light sensitivity.
Accordingly, there is a need and desire for an imager with both excellent low light performance and high dynamic range.